Help for a complete, but keen, beginner

Discussion in 'General Electronics Chat' started by RA23Kev, May 16, 2007.

  1. RA23Kev

    Thread Starter New Member

    May 16, 2007
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    Hi all,

    I am trying set up a simple circuit for turning a piezo buzzer into a one shot beeper. I just want the circuit to get power, make the buzzer beep (0.2s or so) and then when then power resets, beep again. I have tried all sorts of stuff and have failed. I have tried using a cap/res combo and also using a N-channel FET. Not sure if I have the FET wired right (as I am not sure of what I am doing ;-) So if anyone can help can they include how it is wired up as well.

    Many thanks.

    Kev
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Piezo sounders come in two main types, one type requires external circuitry to oscillate while the other type (self-drive) will produce a tone with only the application of power.

    The simpliest solution would be to obtain a piezo with self-drive and then all you should need to activate it is a simple RC scheme to pulse it on at power up.

    What is the part number and who is the manufacturer of the piezo you already have on hand?

    hgmjr
     
  3. RA23Kev

    Thread Starter New Member

    May 16, 2007
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    0
    The piezo I have has the driver inbuilt so when power is applied I get a tone, I just need to have it pulse as a 'beep' and not have a continuous tone.

    The specs are: PC Mount Buzzer 3-30V

    Round PC mount buzzer.

    Specifications:
    - Operating Frequency: 3.8kHz cont
    - Voltage Range: 3 - 30V DC
    - Current Draw: 16mA max
    - Sound Pressure: 85dB/30cm/12V
    - Size Dia: 23.8mm,
    - Height: 16mm
    - Pin Spacing: 15mm.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Here is a sketch a made of a circuit that should do the trick. It uses the n-channel mosfet you mentioned. Pardon the crude drawing.

    I don't know what the Vgs of the mosfet you have is so you may need to tweak the value of the 510K resistor to obtain the sound duration that suits you.

    hgmjr
     
  5. RA23Kev

    Thread Starter New Member

    May 16, 2007
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    Thanks for the diagram. Would this CCT be ok powered by 12V?

    I will rig it up tonight

    Kev
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    It should be ok to power the circuit with 12 volts. The impact of using 12V will be that the piezo will stay on approximately 2 times longer than it would with 5V.

    I expect you will end up adjusting the value of the 510K ohm resistor to obtain the duration of tone you are looking for anyway.

    Let us know how it turns out.

    hgmjr
     
  7. RA23Kev

    Thread Starter New Member

    May 16, 2007
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    The Vgs of the FET is +-20V, do you reckon to go up in resistance if 12V makes it sound longer?
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    The figure of +-20V is more likely the "Absolute Maximum" voltage Vgs. The Vgs needed to fully turn on the mosfet is more likely to be in the range of 3 to 6 volts.

    Can you provide the part number so that we can access a datasheet for the part.

    hgmjr
     
  9. RA23Kev

    Thread Starter New Member

    May 16, 2007
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    0
    It's a P16NF06 NPN FET, your right the gate threshold voltage is 2-4V
     
  10. RA23Kev

    Thread Starter New Member

    May 16, 2007
    6
    0
    Made up the CCT tonight and had no luck, got it to make a very short beep but way too short. Tried substituting some of the resistor values but now it wont work at all so I guess the FET has died. There must be a simple way of getting this to work. The simplest things seem to be the most difficult.

    I will give an overall idea of what I am trying to build. I want a buzzer to sound when a reed switch triggers it, then after 5 sec a buzzer sounds again (I will have a second buzzer to keep it simple). I am combining an off the shelf countdown timer CCT with the buzzer. The idea is that the reed switch sounds the buzzer and also starts the countdown timer that has a relay to energise a second buzzer then the reed switch disconnects. The countdown CCT has a momentary switch that starts the time, if this is held high continuously it doesn't work so I may have to use the CCT here to give it a momentary high or contact signal to start the countdown.

    Kev
     
  11. hgmjr

    Moderator

    Jan 28, 2005
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    Try a larger capacitor. If you have a 2.2 uFd or a 4.7 uFd cap that should get you a longer beep. Even go to a 10 uFd if you need to.

    hgmjr
     
  12. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    The circuit should work. It is not that complicated.

    Do you think you connected the FET up incorrectly at some point and so damaged the device? Don't rule out the possibility that ESD (electro-static discharge) may have destroyed the part.

    hgmjr
     
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